On a front side or a rear side of a vehicle, in order to prevent the impact during collision from being directly transmitted to a passenger, for example, there are many cases where an impact absorption member, such as a crush box, a front side member, or a rear side member, is provided. The impact absorption member is generally composed of a metal material in the related art. As a structure of the metal-made impact absorption member, for example, a structure in which a steel material is press-molded, which is formed in a shape of a box by spot welding, and which is provided with a recessed portion that becomes a starting point of buckling, is generally used (for example, Patent Document 1). In addition, a structure which forms a shape of a box by using an extruded material of aluminum instead of a steel material from the viewpoint of reduction of weight, which is provided with a recessed portion that becomes a starting point of buckling by post-processing, and which is further provided with an attaching portion attached to other components by welding, is known (for example, Patent Document 2). In recent years, in order to improve fuel efficiency, it is desired to reduce the weight of the vehicle, and research on an impact absorption member made of resin has been flourishing (for example, Patent Document 3). Since the resin-made impact absorption member uses a light resin material instead of the metal material in the related art, it is advantageous that substantial reduction of weight of the resin-made impact absorption member can be achieved, and means for achieving further reduction of weight of a vehicle structure is expected.
In the resin-made impact absorption member, in order to obtain stable impact absorption performance, a structure of the impact absorption member is designed so that a load which the impact absorption member receives becomes constant with respect to the modification. For example, in Patent Document 4, a receiving member which has a tapered outer cylindrical surface having a tapered angle, and an impact absorption member fitted to be frictionally engageable with a cylindrical member, are disclosed. In addition, in Patent Document 5, a fiber-reinforced resin-made impact absorption member which changes a sectional shape in the direction orthogonal to the impact absorption direction toward the impact absorption direction, is disclosed. Furthermore, in Patent Document 6, a fiber-reinforced resin-made impact absorption member in which the member thickness on the opposite side to an impact input side is greater than the member thickness on the impact input side, and in which a tip end of an input portion protrudes further outside than an outer circumferential surface, is disclosed. However, there are many cases where the impact absorption members disclosed in each Patent Document have especially complicated structures, and there is a case where a manufacturing method becomes complicated or the use thereof becomes restricted.
Meanwhile, research on obtaining stable impact absorption performance not by making the structure of the impact absorption appropriate, but by making the resin material which constitutes the impact absorption member appropriate, has been carried out. For example, in Patent Document 7, a fiber-reinforced resin-made impact absorption member in which arrangement density of a reinforcing fiber bundle is arranged to gradually increase from the input side to the other side in the impact absorption direction, is disclosed. However, even in this case, there is a case where a special manufacturing method is necessary and a manufacturing process becomes complicated.